Automatically refusing mortgages on properties where Japanese knotweed is found is out of proportion to the risk posed by this invasive species, new research has found.
Ecologists from global infrastructure services firm AECOM and the University of Leeds have carried out the most extensive research to date.
They assessed the potential of Japanese knotweed (Fallopia japonica) to cause structural damage compared to other plants.
Unwelcome visitor
Japanese knotweed is a notorious non-native species in the UK, and elsewhere in Europe and North America.
Now recognised as one of the most problematic weeds in the UK and Ireland, it is known to have a range of negative environmental impacts.
The stigma
When identified in homebuyers’ surveys, mortgage lenders often require evidence that a treatment programme is in place to control Japanese knotweed, entailing significant expense for sellers.
The stigma associated with the plant means that property values can be affected, even after action is taken to control it.
Examining the evidence
The research involved:
Rising shoots of Japanese knotweed (Fallopia japonica) showing how all other plants (such as the bluebell and buttercups in the foreground) are unable to survive in the deep shade of the canopy. This alien plant is becoming an increasing problem all over Britain. Photo credit: Patrick Roper
Dr Mark Fennell, Principal Ecologist at AECOM, who led the study, said: “Our research sought to broaden existing knowledge about the risk to buildings of Japanese knotweed compared to other plants.
“We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity – and certainly no more damage than other species that are not subject to such strict lending policies.”
He added that the seven metre rule, although based on the best information previously available, was not a statistically robust tool for estimating how far the plant’s rhizomes are likely to reach underground.
Co-author Dr Karen Bacon, from the University of Leeds’ School of Geography, said: “The negative impact of Japanese knotweed on such factors as biodiversity and flooding risks remains a cause for concern.
“But this plant poses less of a risk to buildings and other structures than many woody species, particularly trees.
"Japanese knotweed is capable of damaging built structures, but where this occurs, it is usually because an existing weakness or defect has been exacerbated.”
Japanese knotweed is a notorious non-native species in the UK, and elsewhere in Europe and North America.
Now recognised as one of the most problematic weeds in the UK and Ireland, it is known to have a range of negative environmental impacts.
In the UK, Japanese knotweed is widely believed to pose a significant risk of damage to buildings that are within seven metres of the above-ground portions of the plant – the so-called ‘seven metre rule’ – due to its underground shoots, known as rhizomes.
When identified in homebuyers’ surveys, mortgage lenders often require evidence that a treatment programme is in place to control Japanese knotweed, entailing significant expense for sellers.
The stigma associated with the plant means that property values can be affected, even after action is taken to control it.
As well as setting out to test the accuracy of the seven metre rule, researchers examined the risk from multiple lines of evidence. All reached the same conclusion.
The research involved:
- looking for evidence of the perceived threat in previous research literature;
- surveying invasive species control contractors and property surveyors;
- assessing 68 residential properties where Japanese knotweed was found and examined data collected when knotweed was removed by excavation from an additional 81 sites.
Rising shoots of Japanese knotweed (Fallopia japonica) showing how all other plants (such as the bluebell and buttercups in the foreground) are unable to survive in the deep shade of the canopy. This alien plant is becoming an increasing problem all over Britain. Photo credit: Patrick Roper
Dr Mark Fennell, Principal Ecologist at AECOM, who led the study, said: “Our research sought to broaden existing knowledge about the risk to buildings of Japanese knotweed compared to other plants.
“We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity – and certainly no more damage than other species that are not subject to such strict lending policies.”
He added that the seven metre rule, although based on the best information previously available, was not a statistically robust tool for estimating how far the plant’s rhizomes are likely to reach underground.
Co-author Dr Karen Bacon, from the University of Leeds’ School of Geography, said: “The negative impact of Japanese knotweed on such factors as biodiversity and flooding risks remains a cause for concern.
“But this plant poses less of a risk to buildings and other structures than many woody species, particularly trees.
"Japanese knotweed is capable of damaging built structures, but where this occurs, it is usually because an existing weakness or defect has been exacerbated.”
Professor Max Wade, Technical Director – Ecology at AECOM, and co-author of the paper, said: “We hope our research will inform discussions around the advice currently offered about Japanese knotweed by providing more information about the reality of its impact on built structures.”